An example of a communication system related to the invention is described in Non-Patent Literature 1 below. As shown in FIG. 6, this communication system 100 includes a EUTRAN (Evolved Universal Terrestrial Radio Access Network) 110, an MME (Mobility Management Entity) 120, a Serving GW (Gateway) 130, and a PDN (Packet Data Networks) GW 140.
The EUTRAN 110 is a radio access network which provides a communication terminal with connectivity to the communication system 100 using a radio access method referred to as LTE (Long Term Evolution) and includes eNBs (evolved Node B) 111 and 112.
The eNBs 111 and 112 are radio access apparatuses which provide the communication terminal with connectivity to the communication system 100 using LTE. The MME 120 is a control apparatus for performing control so that the communication terminal can receive mobile services via the eNB such as authentication and handover between eNBs.
The Serving GW 130 is an access GW apparatus for providing a bearer that is a communication path for the communication terminal performing data communication, and establishes a tunnel for packet transfer with the eNB and the PDN GW in order to provide the bearer.
The PDN GW 140 is a mobile anchor apparatus which terminates a bearer that is a communication path for the communication terminal performing packet communication, and establishes a tunnel for packet transfer with the Serving GW 130 in order to provide the bearer.
FIG. 7 shows a procedure through which a communication terminal 50 performs handover to the eNB 111 from the state where the communication terminal 50 is attached to the eNB 112 in the communication system 100 shown in FIG. 6. It is assumed that the communication terminal 50 establishes a bearer via the eNB 112 as the initial state.
Referring to FIG. 7, the communication terminal 50 scans a radio wave environment around the communication terminal 50 itself during communication via the eNB 112 and notifies the eNB 112 connected to the communication terminal 50 itself of the scanning result by using a Measurement Report message (d1 in FIG. 7).
After the eNB 112 receives the Measurement Report message and confirms that the radio wave environment of the eNB 111 is better than that of the eNB 112 itself for the communication terminal 50, the eNB 112 determines whether the handover to the eNB 111 is possible or not (Handover Request to Handover Request Ack) (d2, d3 in FIG. 7). When it is possible, the eNB 112 notifies the communication terminal 50 that the communication terminal 50 should perform handover to the eNB 111 [RRC (Radio Resource Control) Connection Reconfiguration] (d4 in FIG. 7).
The communication terminal 50 establishes a link with the eNB 111 in accordance with the instruction from the eNB 112 (RRC Connection Reconfiguration Complete) (d6 in FIG. 7).
When the eNB 111 establishes the link with the communication terminal 50, the eNB 111 switches the bearer for the communication terminal 50 (Path Switching Request to Path Switching Request Ack) (d7 to d10 in FIG. 7).
As described above, in the communication system related to the invention, it is possible to support handover between eNBs by switching the bearer by using the Serving GW 130 and/or the PDN GW 140 as an anchor point.
However, due to an increase in mobile traffic in recent years, a technology called “traffic offloading” has attracted attention. The technology makes it possible to transfer the traffic flowing through the communication system to another communication system as soon as possible by disposing GW apparatuses such as PDN GW, Serving GW and the like, which have been disposed in the inner part of the system in the past, in positions close to the eNB in a distributed manner.
If this distributed arrangement of the GW apparatuses is promoted even further, a communication system in which the GW function is included in the eNB can be constructed. Such a communications system is shown in FIG. 8. This communication system 101 includes a EUTRAN 110 and an MME 120.
Similarly to the communication system 101 shown in FIG. 6, the EUTRAN 110 is a radio access network which provides a communication terminal with connectivity to the communication system 101 using LTE and includes eNBs 111 and 112.
The eNBs 111 and 112 are radio access apparatuses which provide the communication terminal with connectivity to the communication system 101 using LTE. In addition, the eNBs 111 and 112 include GW functions of the Serving GW 130 and the PDN GW 140 shown in FIG. 6.
The MME 120 is a control device for performing control so that the communication terminal can receive mobile services via the eNB such as authentication, inter-eNB handover and the like, as in the case of the communication system 100 shown in FIG. 6.
FIG. 9 shows a procedure through which a communication terminal 50 performs handover to the eNB 111 from the state where the communication terminal 50 is attached to the eNB 112 in the communication system 101 shown in FIG. 8. Because the general procedure in FIG. 9 is the same as that in FIG. 7, only the differences therebetween are shown hereinafter.
It is assumed that as the initial state, the communication terminal 50 establishes a bearer via the eNB 112 by using a Serving GW function and a PDN GW function in the eNB 112.
The communication terminal 50 performs handover from the eNB 112 to the eNB 111 in the same procedure as that shown in FIG. 7. In this case, it is also possible to support handover between eNBs in this system by continuously using functions of the eNB 112 as the Serving GW function and the PDN GW function that serve as anchor points (e1 to e 10 in FIG. 9).